A conventional method of producing a carbonic acid diester by a reaction of phosgene with an alcohol is a well known and very old method that has been practiced for some time. Nevertheless, this conventional method is disadvantageous in that phosgene has an extremely strong toxicity, and thus is not preferable as a starting material in view of environmental and health considerations. Also, since the reaction of the conventional method produces hydrochloric acid as a by-product, which corrodes the reaction device, an expensive material must be selected and employed for the reaction apparatus.
Therefore, there is a strong demand for a new method of producing a carbonic acid diester without using phosgene, in consideration of the environmental and health and industrial difficulty.
In response to this demand, various attempts have been made to produce a carbonic acid diester from an alcohol and carbon monoxide, as disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 60-75,447, and 63-72,650, and Japanese Examined Patent Publication (Kokoku) 63-38,018.
In those methods, the carbonic acid diester is produced by a catalytic oxygen-oxidizing reaction of carbon monoxide with an alcohol in a liquid phase, in the presence of a catalyst consisting of a copper halide or palladium halide. These methods are disadvantageous in that, in the catalytic oxygen-oxidizing reaction, carbon dioxide is produced as a by-product, and thus the production of the carbonic acid diester is effected with a low selectivity based on the amount of carbon monoxide supplied to the reaction system, and the catalytic oxygen-oxidizing reaction produces water as another by-product, and thus the isolation of the resultant carbonic acid diester is difficult. Further, the methods as disclosed in the above-mentioned publications are not always industrially advantageous in that the reactions of the conventional methods are liquid phase reactions and thus a procedure for separating the resultant product from the catalyst is necessary.
There have been attempts made to eliminate the above-mentioned disadvantages, and as one such attempt, Japanese Unexamined Patent Publication (Kokai) No. 60-181,051 discloses a method of producing carbonic acid diester by a catalytic oxidizing reaction of a nitrous acid ester with carbon monoxide, in a gas phase, in the presence of a catalyst composed of a solid platinum group metal or compound thereof carried on a solid carrier and an oxidant in an amount of 10 molar % in terms of O.sub.2, per mole of carbon monoxide present in the reaction mixture.
This method however, is disadvantageous in that the oxidant in the above-mentioned specific amount based on the carbon monoxide effectively inhibits a production of oxalic acid diester as a by-product, but the addition of the oxidant in the above-mentioned specific amount based on the carbon monoxide cannot completely inhibit the production of the oxalic acid diester, and therefore, the target carbonic acid diester is produced with an unsatisfactorily low selectivity, and the reaction rate and the durability of the catalyst are unsatisfactory.
Further, the above-mentioned method is further disadvantageous in that the proportion of nitrous acid ester in a reaction mixed gas comprising the nitrous acid ester, carbon monoxide, alcohol and oxygen is higher than an explosion (flammable) limit of the mixed gas, and thus this reaction mixed gas is not industrially satisfactory in view of the safety of the procedure.
Japanese Unexamined Patent Publication (Kokai) Nos. 3-141,243 and 4-139,152 disclose a method of producing dimethyl carbonate by a catalytic reaction of carbon monoxide with methyl nitrite in a gas phase by using a catalyst comprising a carrier, for example, activated carbon, and a catalytic member selected from compounds of platinum group metals, for example, palladium chloride and palladium sulfate, and compounds of metals selected from iron, copper, bismuth, cobalt, nickel and tin, and carried on the carrier. This method is unsatisfactory in catalytic activity and durability of the catalyst for practical use in industry.
As mentioned above, the conventional methods of producing carbonic acid diester by using a nitrous acid ester are unsatisfactory in the durability of the catalyst and are not always satisfactory in reaction rate and selectivity of the carbonic acid diester.